lkml.org 
[lkml]   [2022]   [Jul]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] blk-mq: avoid potential infinite loop in __blk_mq_alloc_request
From
> On Fri, Jul 22, 2022 at 01:22:23PM +0800, Liu Song wrote:
>> From: Liu Song <liusong@linux.alibaba.com>
>>
>> If "blk_mq_get_tag" returns BLK_MQ_NO_TAG because the value of
>> "tags->nr_reserved_tags" is 0, it will fall into an infinite loop in
>> "__blk_mq_alloc_requests", so borrow BLK_MQ_REQ_NOWAIT to exit the loop.
> That means the driver calling blk_mq_alloc_request has a bug, and
> we should not work round that in the low level tag allocation path.
>
> If we want to be nice we can add a WARN_ON before going all the way
> down into the tag allocator, something like:
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 92aae03103b74..d6c7e2ece025f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -520,6 +520,10 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
> struct request *rq;
> int ret;
>
> + if (WARN_ON_ONCE((flags & BLK_MQ_REQ_RESERVED) &&
> + !q->tag_set->reserved_tags))
> + return ERR_PTR(-EINVAL);
> +

Hi,

It is a reasonable approach to prevent abnormal alloc from going down,
but this is a very rare exception after all, and above modification is
checked
every alloc request, which seems to be a bit excessive overhead.

After the rare exception occurs and fix it, the impact on the alloc
request will be lower.

Thanks

> ret = blk_queue_enter(q, flags);
> if (ret)
> return ERR_PTR(ret);

\
 
 \ /
  Last update: 2022-07-22 10:17    [W:0.061 / U:0.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site